Progestogens for maintenance tocolysis in symptomatic women. A systematic review and meta-analysis

Objective Prevention of preterm birth (PTB) with progestogens after an episode of threatened preterm labour is still controversial. As different progestogens have distinct molecular structures and biological effects, we conducted a systematic review and pairwise meta-analysis to investigate the individual role played by 17-alpha-hydroxyprogesterone caproate (17-HP), vaginal progesterone (Vaginal P) and oral progesterone (Oral P). Methods The search was performed in MEDLINE, ClinicalTrials.gov and the Cochrane Central Register of Controlled Trials (CENTRAL) up to 31 October 2021. Published RCTs comparing progestogens to placebo or no treatment for maintenance tocolysis were considered. We included women with singleton gestations, excluding quasi-randomized trials, studies on women with preterm premature rupture of membrane, or receiving maintenance tocolysis with other drugs. Primary outcomes were preterm birth (PTB) < 37 weeks’ and < 34 weeks’. We assessed risk of bias and evaluated certainty of evidence with the GRADE approach. Results Seventeen RCTs including 2152 women with singleton gestations were included. Twelve studies tested vaginal P, five 17-HP, and only 1 oral P. PTB < 34 weeks’ did not differ among women receiving vaginal P (RR 1.21, 95%CI 0.91 to 1.61, 1077 participants, moderate certainty of evidence), or oral P (RR 0.89, 95%CI 0.38 to 2.10, 90 participants, low certainty of evidence) as opposed to placebo. Instead, 17-HP significantly reduced the outcome (RR 0.72, 95% CI 0.54 to 0.95, 450 participants, moderate certainty of evidence). PTB < 37 weeks’ did not differ among women receiving vaginal P (RR 0.95, 95%CI 0.72 to 1.26, 8 studies, 1231 participants, moderate certainty of evidence) or 17-HP (RR 0.86, 95%CI 0.60 to 1.21, 450 participants, low certainty of evidence) when compared to placebo/no treatment. Instead, oral P significantly reduced the outcome (RR 0.58, 95% CI 0.36 to 0.93, 90 participants, low certainty of evidence). Conclusions With a moderate certainty of evidence, 17-HP prevents PTB < 34 weeks’ gestation among women that remained undelivered after an episode of threatened preterm labour. However, data are insufficient to generate recommendations in clinical practice. In the same women, both 17-HP and vaginal P are ineffective in the prevention of PTB < 37 weeks’.

related to "tocolysis", "preterm labour" and "17-alpha-hydroxyprogesterone caproate" "vaginal progesterone", "oral progesterone" from 1966 until 31 October 2021. To locate additional publications, we reviewed bibliographies of identified studies and reviews articles. No restrictions for language or geographic location were applied. The detailed search strategy is reported in the Supplementary Material (S1 Table).

Study selection
We included randomized controlled trials on singleton gestations that remained undelivered after an episode of preterm labour, and were then randomized to maintenance tocolysis with either 17-HP, oral P or vaginal P as opposed to no treatment or placebo. Threatened preterm labour was homogenously defined, in the different studies, as the simultaneous presence of regular contractions and cervical changes. Exclusion criteria included quasi-randomized trials, maintenance tocolysis in women with preterm premature rupture of membranes, and maintenance tocolysis with other drugs. Two reviewers (F.F., F.F.) independently screened each record retrieved based on titles and abstracts. Potentially relevant studies were acquired in full text and independently assessed for final inclusion by two authors (F.F., F.F.). Any disagreement was discussed with a third author (S.M.).

Data extraction and analysis
Data abstraction was independently completed by the same 3 investigators. Any disagreement was reviewed and further resolved by discussion. Data abstracted included: number and characteristics of participants (age, parity, cervical length, history of PTB), number of participants and mean gestational age at randomization in the intervention and control groups, frequency of administration and dosages of 17-HP, vaginal P or oral P.
In case of missing data on the primary outcomes, we contacted study Authors allowing them one month to reply. Two authors (F.F., S.M.) independently assessed risk of bias according to the criteria set out in the Cochrane Handbook for Systematic Reviews of Interventions [14]. The following criteria were considered: sequence generation and allocation concealment (selection bias), blinding of participants and providers (performance bias), blinding of outcome assessors (detection bias), incomplete outcome data (attrition bias), and selective outcome reporting (reporting bias). Disagreement between reviewers was resolved by discussion.
We analyzed dichotomous outcomes by calculating the risk ratio (RR) for each trial with the uncertainty in each result being expressed with a 95% confidence interval (CI). We analyzed continuous outcomes by calculating the mean difference (MD) with 95% CI. In case the number of analyzed participants did not correspond to the number of randomized subjects, we assumed that missing participants were missed at random, i.e. that missing was unrelated to actual values of the missing data. Therefore, we included in the analysis only the available data without any imputation. As we hypothesized a certain degree of heterogeneity among studies due to treatment schedules, criteria of assessing response, risk of bias and other factors which may have affected direction and magnitude of treatment effect, we pooled data using the random effect model for each outcome. Seeking statistical heterogeneity among studies, the Cochrane Q-test was used with a significant threshold of alpha = 0.1 and inconsistency among studies was quantified by the I-squared statistic [14]; an I square >70% was considered as significant heterogeneity. We planned to perform subgroup analysis for known risk factor of preterm delivery: cervical length (with a cut-off of 25 mm), and previous PTB or late pregnancy loss (with a cut off of 20% of participants with history of PTB). Results are depicted in all figures as conventional meta-analysis forest plots. RevMan 5.4 was used to generate forest plot figures (Review Manager, version 5.4.1 Copenhagen; 2020). We planned to use visual inspection of funnel plots (plots of the effect estimate from each study against the sample size or standard error) to indicate possible publication bias if there were at least 10 studies included in the meta-analysis.

Grading of evidence
We assessed the overall quality of the evidence for the primary outcomes using the five GRADE domains (study limitations, consistency of effect, imprecision, indirectness, and publication bias) according to the GRADE approach [15]. Based on the above domains, the GRADE system uses the following rating to grade the evidence: High: we are very confident that the true effect lies close to that of the estimate of the effect. Moderate: we are moderately confident in the effect estimate: the true effect is likely to be close to the estimate of the effect, but there is a possibility that it is substantially different. Low: our confidence in the effect estimate is limited: the true effect may be substantially different from the estimate of the effect. Very low: we have very little confidence in the effect estimate: the true effect is likely to be substantially different from the estimate of effect. The existing evidence was summarized in a "Summary of Findings" table (S2 Table) that provides key information about the magnitudes of relative and absolute effects of the interventions, the amount of available evidence and the certainty of available evidence. We used GRADEproGDT software (GRADEpro GDT, McMaster University; 2015).

Results
After duplicates were removed, a total of 4390 records were found. We acquired 21 articles in full text, as potentially relevant. Four studies were excluded: 2 because they were not RCTs [16,17], 1 because participants and interventions did not match the inclusion criteria [18], and 1 because we were unable to retrieve the full text [19] (Fig 1 [13]).
Six studies were funded by public institutions [26,28,29,31,33,34]; the remaining did not report their funding source. Table 1 summarizes the characteristics and results of each trial.

Risk of bias of included studies
Seven studies were considered at low risk of selection bias because random sequence generation and allocation concealment were appropriate [9,24,26,28,29,31,33]. Furthermore, random sequence generation was appropriate in 7 [21,22,25,27,30,32,34] studies but information on allocation concealment was not provided. The work of Areia et al was judged at high risk of bias for allocation concealment [20], while Briery et al did not describe how random sequence was generated despite adequate allocation concealment [29]. Eight studies were considered at high risk of performance bias because they were open label [20,22,25,26,30,32,33], while the remaining were judged at low risk as they were placebo controlled, in a double blind design. All the included RCTs were judged at low risk of detection bias because the study outcomes were objective and unlikely to be biased by lack of blinding. Two RCTs were rated high for risk of attrition bias [27,28]. The study protocol was available only for five studies   [9,26,29,31,33], where the outcomes reported in the final publication coincided with the ones listed in the original protocol; for taball the remaining studies the protocol was not available, therefore they were considered at unclear risk of selective outcome reporting bias (Fig 2). Table) PTB < 34 weeks. Preterm birth <34 weeks' gestation did not differ among women receiving vaginal P as opposed to placebo/no treatment (RR 1.21, 95%CI 0.91 to 1.61, 7 studies, 1077 participants, moderate certainty of evidence), or receiving oral P as opposed to placebo (RR 0.89, 95%CI 0.38 to 2.10, 1 study, 90 participants, low certainty of evidence). Instead, 17-HP significantly reduced the outcome when compared to placebo/no treatment (RR 0.72, 95%CI 0.54 to 0.95, 4 studies, 450 participants, moderate certainty of evidence) (Fig 3).
Subgroup analysis. Subgroup analyses for mean cervical length at baseline was not feasible, as 5 studies did not report the measure and all but 2 of the remaining studies reported a mean cervical length less than 25 mm.
Subgroup analysis for history of PTB/late pregnancy loss was possible only when vaginal P was compared to placebo/no treatment (6 studies), while only 1 study investigating 17-HP reported this information. No significant differences were found in the incidence of PTB < 34-or < 37-weeks' gestation (S20 and S21 Figs respectively) between subgroups of studies including > or � 20% of participants with a history of PTB/late pregnancy loss (Test for subgroup differences: Chi 2 = 1.04, df = 1 (P = 0.31), I 2 = 4.3% and Chi 2 = 0.15, df = 1 (P = 0.70), I 2 = 0%, respectively).

Discussion
This systematic review is the largest up-do-date, including 17 RCTs and 2152 singleton gestations, 410 (23%) women more than previous meta-analysis [9].
The quantitative meta-analysis showed for the first time, with moderate certainty of evidence, that 17-HP given to women remaining undelivered after an episode of threatened preterm labour significantly reduced their risk of PTB <34 weeks' gestation. It should be underlined that the relatively small number of events impacts on the precision of the estimate. We considered PTB <34 weeks' gestation as the primary outcome because clinical management of threatened preterm labour substantially differs after this time point. Acute tocolysis is not recommended in the late preterm period [36] (i.e. delivery between 34 and 37 weeks') [37] while antenatal corticosteroids treatment is still debated [38].
Although our primary outcome had been addressed in previous meta-analyses [9,39], our conclusions differ due to a larger study population, that now includes data recovered from 2RCTs [31,32] originally reporting PTB <35 weeks' as primary outcome. This allows almost doubling the number of participants in our actual study. Despite such positive effect on PTB <34 weeks' gestation, we confirmed that 17-HP did not affect PTB <37 weeks'. The apparent inconsistency may be due to a limitation of primary studies, which do not systematically differentiate between spontaneous and iatrogenic PTB (defined as planned delivery for maternal and/or adverse fetal conditions), the latter being the most common delivery cause in the late preterm period [37].
Based on a large study population (>2000 women) deriving from different countries, our meta-analysis also corroborated the previous suspect that vaginal P is not helpful for PTB prevention after an episode of preterm labour [9,39,40]. Nationality represents a pivotal issue, as a secondary analysis of the "4P trial" demonstrated that vaginal P increased the risk of PTB in women from Switzerland while it was ineffective among Argentinians [41], hinting towards potential harmful effects, as already hypothesized by other Authors [9].
Distinct molecular structures, biological effects and sites of actions (genomics and nongenomics) may account for differences in the effectiveness of P and 17-HP on PTB rates [42,43]. Accordingly, we decided to analyse RCTs using the 2 progestogens separately, and not to combine data on vaginal and oral P, as the 2 routes of administration lead to different metabolites and clearance [44]. Thus, we cannot drew definitive conclusions on oral P since only one, low-quality small trial was included in our review.
Concerning secondary outcomes we confirmed that both 17-HP [11] and vaginal P [10] are associated with prolongation of pregnancy when compared to placebo/no treatment, leading to higher birth-weights, despite unchanged NICU admission rates. The fact the vaginal P treatment is not associated with reduction of PTB < 34 weeks but with significantly increased latency could be related to the gestational age at randomization. Indeed, vaginal P studies randomized women almost two weeks later compared to 17-HP studies. Latency is therefore an outcome conditioned by gestational age at randomization, so it must be interpreted in relation to it. Moreover, the three trials that reported a significant latency did not investigate PTB <34 weeks as an outcome [22,33,34]. Finally, it also seems that perinatal mortality and RDS were reduced among women treated with vaginal P, while data on 17-HP showed no difference [11]. The apparent conflicting results between effects of the 2 progestogens on primary outcomes and lack of related effects on morbidity, could be ascribed to the small number of events included in the MA for low birth weight and RDS. The small number of events, together with the small sample size and the relatively small number of included studies strongly affect the precision of the estimate, returning in wide confidence intervals crossing the line of no effect.
The strengths of this systematic review include its comprehensive search strategy and rigorous statistical analysis. Our findings do not solely rely on the higher number of included studies, but also on the systematic search of unpublished data for the two primary outcomes. We performed a subgroup analysis on the impact of additional risk factors for PTB, such as previous PTB/late pregnancy loss, showing for the first time that vaginal P did not affect PTB rates, independently of patients' obstetric history. Furthermore, we found a trend towards higher PTB rates < 34 weeks' gestation when vaginal P was given to populations with previous PTB.
Our work is not without limitations: different potential sources of risk of bias, as well as lack of availability of the majority of study protocols, increased the risk of selective outcomes reporting. This is the reason why we drew definitive conclusions only on the primary outcomes. We could not perform the subgroup analysis on the history of PTB/late pregnancy loss on women treated with 17-HP due to the paucity of studies. Similarly, subgroup analysis for mean cervical length at baseline was not feasible because 5 studies did not report the measure, and all but 2 of the remaining studies reported a mean cervical length < 25 mm. These limitations remind us of the importance of international networks to encourage database exchanges. For instance, the recent Individual Patient Data meta-analysis on the effects of progestogens in women at risk for positive obstetric history or asymptomatic short cervix had such a large population that its conclusions could impact clinical practice [45].
In conclusion, with a moderate certainty of evidence we found that 17-HP prevents PTB < 34 weeks' gestation among women that remained undelivered after an episode of threatened preterm labour. Vaginal P was ineffective, although it seems to prolong pregnancy. However, there are insufficient data to generate recommendations for the use of progestagens as a maintenance tocolytic in clinical practice. Prior to any change in patients' management, a large multicentre international RCT is urged to detect possible differences in PTB prevention among women treated with vaginal P, intramuscular 17-HP or placebo.